Method of and means for controlling light



' Sept- 27" 1932- w. H. `EcicLEs ET-Al. 1,879,138

METHOD OF AND MEANS FOR CONTROLLING LIGHT Filed Oct. 25, 1928 property,

cells, for controlling applied .voltage and, to only a directly proportional to the appliedrvoltage.

Patented Sept. 27, 1.932

UNITED STATES PATENT ori-ica WILLIAM HENRY ECOLES, 0F LONDON,

ENGLAND, ASSIGNOBS,

AND ALFRED WHITAKE'B, 0F WEST DRAYTON, BY MESNE ASSIGNMENTS, T0 RADIO CORPORATION OF AMERICA, OF NEW YORK, N. Y., A CORPORATION OF DELAWARE METHOD OF AND MEANS FOR CONTBOLLING LIGHT Application fled October 25, 1928, Serial No. '815,048, and in Great Britain October 29, 192;?.

rIhe present invention relates to means for controlling the intensity of a beam of light iii accordance with electrical oscillations.

It 'is known'that certain bodies have the when subjected to electro-static stress, of becoming doubly refracting or, if they are initially doubly refracting, of having this property modified in' degree. This eect has been utilized in light cells, known as Kerr the'intensity of a beam of light in accordance with electrical oscillations.` The light is passed through two polarizing'devices between which is arranged the electro-statically influenced body. The degree of extinction of the light passing through the system is thus varied in accordance with the electrical variations impressed upon .the device. It has also been proposed to utilize, as the body subjected to electro-static stress in modulating a beam of light, a piezo-electric crystalline body, alternating voltages being applied to the body to cause it to oscillate. One disadvantage which is experienced in using this arrangement, or any known form of Kerr cell. in making a photographic record of sounds isthat the rotation produced is, in the main, proportional to the square of the small extent,

It has been discovered by Pockels that certain acentric crystals, which exhibit the Kerr or second order effect, also exhibit another effect which is of much greater magnitude than thel second order eifect and which is a first order effect, that is to say an eiect in which the resultant change in illumination is directly proportional to the applied voltage, This eifect will be referred to as the Pockels effect. According to the present invention, the intensity of a beam of light is modulated by passing the light through a material in which the` Pockels eiect preponderates over the second order effect, voltage variations being appliedfto the material. The relation between tbe applied voltage and the change in illumination produced is therefore substantially linear. Rochelle salt has, for example, been found to be/suitable.

The invention is illustrated in the accompanying drawing in which allel also to the direction I the system 6, l,

Fig. lis a view in sectional elevation of a cell in accordance with the invention.

Fig. 2 is a diagram illustrating the appli,- cation of the invention to the recording of sound. A

Fig. 3 is a view of a crystal suitable for use in connection with the present invention, and

Fig. 4 is a view, similar to Fig. 1, of a modified formof our invention.

Referring to Fig. 1, a piece 1 cut from a singlecrystal of Rochelle salt, preerably in a manner to be described later, is mounted between two co-aXia-l-cylinders 2 of insulating material, the ends of these cylinders being closed with glass discs 3. Uponthe inside of each of these cylinders is arranged an annular ring 4 of conduct-ing material, connecting wires 5 being taken from each of these rings through the insulating tubes to the outside. The conducting rings 4 act as the electrodes of the device. The space within the two tubes is illed with some feebly conducting liquid L which does not dissolve the crystal, such, for example as alcohol. The parts of the device may be fixed together in any suitable manner.

It will be noted that the lines of electrostatic orce, when a difference of potential isf lmaintained between the two electrodes 4, will run approximately parallel to the axis of the tubes 2 and that this direction is parin which light, paing in through one glass disc and out through the other, is transmitted through the cell.

f InV the alternative device illustrated in Fig. 4, the electro-static lines of force are at right angles tothe direction of the light. The crystal 1 is, in this case, arranged between two parallel conducting plates 14, the light bein transmitted in a direction parallel to the sur aces of these plates.

In using cells of this type `for controlling the intensity of a beam of light, the cell may be arranged, as shown in Fig.,2, between two polarizing devices 6, and the potential variations, which are to be converted into changes of intensity of the beam of light, are applied to the two electrodes through the conductors 5. The light from source 11 passes through 6 and falls through asuitable ing sound, maybe either velectrical able slit 12 upon a photographic lm 13. The degree of extinction ofthe light beam will thus be varied in accordance with the applied potentials.

The light used, may, if desired, be monochromatic. Either a polarizing direct voltage may be provided in circuit with the normal working electrodes, such as a battery 16, or, alternatively, auxiliary electrodes (not shown) may be employed for obtaining a. polarizing electro-static stress. The polarizing electro-static stress may be in the same or in a different direction from the varying electro-static stress. If desired, the whole or part of the light may be caused, by suitable optical means, to traverse the cell repeatedly in such direction as to augment the effect.

The electrical variations applied to the electrodes of the cell, for example in recordvariations corresponding to the acoustic vibrations or they may behigh frequency oscillations modulated with the acoustic vibrations.

Fig. 3 illustrates the preparation of a suitcrystal of Rochelle salt. The crystal from which the working material is to be cut should be one exhibiting the surface markingindicated at 7, which is known as the hour-glass structure. The crystal is cut along a plane indicated by the dotted lines 8 and the lower part is rejected. The pieces which will best serve for light modulation are those within the dotted lines 9 and 10, the pieces 9 being, if anything, preferable.

We claim:

p l. The method of controlling the intensity of a beam of light by means of a material exhibiting the Pockels effect which consists in passing said beam of light through said material along an axis in which the Pockels effect preponderates over the second order effect and applying voltage variations to said material.

2. The method of controlling the intensity of a beam of light by means of a material exhibiting the Pockels effect and in accordance with voltage variations which consists in passing said beam of light in succession through a polarizing device, along an axis of said material in which the Pockels effect preponderates over the second order effect, and a second polarizing` dev ice, said voltage variations being applied to said material.

3. The method 'as claimed in claim 1 Wherein said voltage variations are in the form of modulated high frequency oscillations.

4. The method as claimed in claim 1 wherein said voltage variations are in the form of modulated high frequency oscillations and wherein said high frequency oscillations have a frequency corresponding substantially to the natural frequency of said material or to a harmonic thereof.

5. The method as claimed in claim 1 wherel wherein the Pockels effect in a 4steady direct voltage is superposed upon said voltage variations.

6. The method as claimed in claim 1 in wlliich said material is a crystal of Rochelle sa t.

7 In apparatus for controlling the intensity of a eam of polarized light, the combination of a crystal, means for directing said beam of light therethrough, and means for applying a variable voltage thereto, said crystal being a section of a crystal of the type exhibiting hour glassmarking and being cut adjacent said marking.

8. In apparatus for controlling the intensity of a beam of olarized light, the combination of a crysta means for directing said beam of light therethrough, and means for applying a variable voltage thereto, said crystal being of the type exhibiting hour glass marking and being so disposed that said light beam is passed through said crystal-adjacent said marking.

' 9. In apparatus for controlling the intensity of a beam of polarized light the combinationof a Rochelle salt crystal, means for directing said beam of light therethrough, and means for applying a variable voltage thereto, said crystal exhibiting hour glass mark` ing and being so disposed that said light beam is passed through a portion of said marking. V

l0. The method of controlling the intensity of a beam of light by means of a crystal exhibiting the Pockels effect which consists in passing said beam of light through said crystal along an axis in which the Pockels e ect predominates over the second order effect and applying voltage variations to said crystal.

11. The method of controlling the intensity of a beam of light by means of a Rochelle salt crystal wherein the Pockels effect is eX- hibited which consists in passing said beam of light through said crystal along anaxis predominates over the second order effect and applying voltage variations to said crystal.

l2. The method as claimed in claim l, wherein said voltage variations correspond to acoustic vibrations.

In testimony whereof,

names.

WILLIAM ALFRED we have signed our HENRY EcoLEs. WHITAKER. 

